1. Technical Field
The present invention relates generally to an image forming apparatus having an improved structure wherein a manual paper feed inlet is formed in an upper surface of a casing in addition to a recording sheet storage cassette.
2. Background Art
In recent years, there have been proposed image forming apparatuses such as a copying or printing machine which have either a cassette paper feed system for feeding a recording paper sheet from a paper cassette or a manual paper feed system not using a paper cassette.
Hereinafter, an example of a conventional image forming apparatus will be described with reference to FIGS. 6 and 7.
The shown image forming apparatus includes a photosensitive medium 2, an electrifier 4, an exposure optical system 5, a developing roller 6, a transfer roller 7, and a cleaning blade 8.
The photosensitive medium 2 is formed with a thin film, made of selecium (Se) or organic photoconductive (POC) materials, applied on an outer peripheral surface of a drum which is rotatably supported by a shaft 3 mounted on a side wall of a process unit 1. The electrifier 4 is connected to a high voltage power supply (not shown), and includes an electrifying blush 4a made of conductive rayon and an electrifying plate 4b made of a metallic plate. Applying a high voltage on the order of 1.2 kV to the electrifying plate 4b causes the electrifying bush 4a to discharge so that the photosensitive medium 2 is charged uniformly. The exposure optical system 5 emits an exposure beam 5a onto the photosensitive medium 2 to form electrostatic latent images according to image signals. The developing roller 6 is composed of a conductive member 6b made of urethane formed around a developing roller shaft 6a, and is supported rotatably on the side wall of the process unit 1. Similarly, the transfer roller 7 includes a conductive member 7b made of urethane formed on an outer peripheral surface of a transfer roller shaft 7a rotatably supported on the side wall of the process unit 1. The cleaning blade 8 serves to remove unnecessary toner left on the photosensitive medium 2.
A transfer paper cassette 9 storing therein a stack of sheets of transfer sheet 10 is disposed on the bottom of the image-forming apparatus. The transfer sheet 10 is fed by a paper pickup roller 11, in sequence, to a transfer sheet feeding path 13 through a nip formed between the paper pickup roller 11 and a feeding roller 12. A manual paper feed inlet 14 is formed in an end wall of a casing 50. When an operator inserts the transfer sheet 15 into the manual paper feed inlet 14 from the direction B, as shown in FIG. 6, it is fed under feeding pressure exerted by the paper pickup roller 11 and the feeding roller 12 toward the transfer sheet feeding path 13.
The image forming apparatus further includes a registry arm 17, a fixing device 18, and a selector member 21. The registry arm 17 is provided to bring the transfer sheet 15 into registration with toner images formed on the photosensitive medium 2. The fixing device 18 is composed of a heat roller 19 incorporating therein a heat source and a pressure roller 20. The heat roller 19 engages the pressure roller 20 to form a nip through which the transfer sheet 10 passes under heat and pressure for fixing the toner images transferred on the transfer sheet 10. The selector member 21 is pivotably mounted to assume two positions: a face-up feed position, as indicated by a broken line, and a face-down feed position, as indicated by a solid line. The selector member 21 feeds, in the face-down feed position, the transfer sheet 10 toward face-down rollers 22, while it feeds the transfer sheet 10 toward face-up rollers 23 in the face-up feed position.
In operation, a high voltage of the order of -1.2 kV is first applied to the electrifier 4 through the high voltage power supply to uniformly charge the surface of the photosensitive medium 2 rotating in the direction, as indicated by an arrow in FIG. 7, at a voltage on the order of -700 V. The exposure beam 5a in the form of a laser beam is then radiated according to image data onto the charged surface of the photosensitive medium 2 to form electrostatic latent images thereon. The timing to start forming the electrostatic latent images is determined based on a signal provided by the registry arm 17. Subsequently, a negative voltage of the order of -300 V is applied to toner deposited on the surface of the developing roller 6. This will cause the toner on the developing roller 6 to be transferred only to portions of the photosensitive medium 2 from which the charge is removed by the radiation of the exposure beam 5a to perform the development in the so-called negative-positive process. The transfer roller 7 is applied with a high voltage of the order of +1 kV to transfer the toner deposited on the photosensitive medium 2 to the surface of the transfer sheet 10 transported by the feeding roller 12.
When the transfer sheet 15 is manually inserted into the manual paper feed inlet 14, it is transported to the transfer roller 7 according to rotation of the paper pickup roller 11 and the feeding roller 120 so that the toner deposited on the photosensitive medium 2 is transferred on the transfer sheet 15. The timing to start forming electrostatic latent images on the photosensitive medium 2 is, similar to the above, determined based on a signal provided by the registry arm 17. For stable feed of the transfer sheet 15, a manual paper feed tray 16 is provided on the side wall of the casing 10 for preventing the transfer sheet 15 from being inserted obliquely into the manual paper feed inlet 14. The unnecessary toner left on the photosensitive medium 2 without being transferred to the transfer sheet 10 or 15 is removed by the cleaning blade 8.
Afterwards, the toner image-transferred transfer sheet 10 or 15 is fed to the fixing device 18 wherein the toner images are fixed under heat of the heat roller 19 and pressure exerted by the pressure roller 20, after which it is directed by the selector member 21 to the face-up rollers 23 or the face-down rollers 22.
The above discussed prior art image forming apparatus has suffered from the following drawbacks. Usually, into the manual paper feed inlet 14, a special size of the transfer sheet 15 such as an OHP sheet, an envelope, or a transfer sheet for double side printing is manually inserted. Thus, for stable feed of the transfer sheet into the manual paper feed inlet 14, the manual paper feed tray 16 having a size enough to receive the transfer sheet 15 is needed. This, however, results in the overall structure of the image forming apparatus becoming bulky. Additionally, if the manual paper feed tray 16 having a size much smaller than that of the transfer sheet 15 for reduction in overall size of the apparatus, the transfer sheet 15 may be inserted obliquely into the manual paper feed inlet 14.